I. Field of the Invention
The present invention relates to the preliminary pressing of glass plastic assemblies. Glass plastic assemblies comprise two or more glass sheets with an interlayer of transparent plastic material between each pair of glass sheets. Although not limited in the present invention, the term "glass" also means optically transparent sheets of plastic materials, such as acrylic and polycarbonate plastics, that are often substituted for glass. The term "interlayer" comprises thermoplastic materials such as polyurethanes and plasticized polyvinyl butyral that bond glass sheets together.
The present invention, more particularly, relates to the preliminary pressing of assemblies comprising bent glass sheets, particularly those that have portions that are deemed critical to laminate together, such as sharply bent portions in an assembly of glass sheets bent to nonuniform radii of curvature.
II.A. Technical Problems
Laminated glass sheets are commonly used in cases where safety glass is required, such as in windshields for vehicles and laminated windows. The ability of the interlayer of thermoplastic material to bond the glass sheets together prevents the individual glass sheets from flying about in case of impact and also, the flexibility of the interlayer sheet provides a yielding safety device that prevents the head of an occupant of the vehicle from penetrating the windshield in case of an accident.
To achieve a satisfactory laminated windshield or other laminated product, it is necessary that the glass sheets and plastic interlayer bond together over the entire area of the glass sheets, and it is also necessary that the bonding be such that the delamination does not occur in use. To achieve this desired result, the plastic interlayer must fill the entire space between the glass sheets, and also for the glass sheets to be bonded together over their whole area during the laminating process.
This object of filling the entire space between the glass sheets is difficult to attain, particularly when the glass sheets forming the assembly are shaped to nonuniform curves. In such instances, critical regions are formed in the sharply bent portions of the glass sheets where even a slight displacement or misalignment between the two bent sheets as they are assembled with a sheet of thermoplastic interlayer material therebetween makes it difficult to avoid the breakage of the glass or the formation of bubbles entrapped within the interfacial surfaces between the glass sheets and thermoplastic interlayer sheet.
Several techniques have been suggested and used to laminate bent glass plastic assemblies. In one of these, the assembly is heated to a condition wherein the interlayer is softened, and the assembly passed between a pair of rotating nipper rolls which seal the edge of the glass and force entrapped fluid from between the interfaces beyond the assembly. However, the more complicated the shapes to which glass sheets are bent, the more difficult it is to avoid breakage of the glass sheets as the assemblies are roll pressed between the rotating rolls. In another known process, the assembly is inserted within a flexible envelope which is evacuated and sealed and then the assembly together with the envelope is subjected to elevated heat and pressure. This well known process requires very careful handling to ensure proper insertion of the glass within the envelope or bag and expensive bagging equipment to perform the preliminary pressing operation. When glass sheets are laminated inside plastic bags, some breakage occurs as a result of inserting the assemblies within the bags. Furthermore, evacuation inside the plastic bag pulls the glass sheets against the plastic interlayer when the air is removed and misalignments between the sharply bent areas of the glass sheets would result in breakage. In either case, it is impossible to detect the breakage until the bag and its contents are subjected to a final pressing treatment and the contents removed from the bag. This means that equipment used for the final lamination of assemblies was wasted on assemblies that were already broken. This prevented the maximum utilization of the autoclave equipment used for final lamination.
Still another preliminary pressing method is accomplished by mounting an open ring-like member of fluid-impervious material with its lips engaging the major surfaces of the assembly and its base spaced from the perimeter of the assembly to form an evacuation channel. The evacuation channel is fluid impervious and is connected through a pipe to a source of subatmospheric pressure. The latter type of preliminary pressing operation is highly labor intensive and also requires a special inventory of ring-like members for each production pattern having a unique outline shape and bend.
It would be beneficial for the glass laminating art to develop a technique that eliminates the need for either pressing rolls or plastic bags or plastic evacuating rings forming peripheral evacuating chambers around the assembly.
II.B. Description of Patents of Interest
U.S. Pat. No. 2,965,527 to Morris discloses a method of pressing curved laminated glass assemblies in which an assembly is inserted within a chamber containing air, and air is evacuated from the chamber at the onset of the operation. Sometimes, the shape of the assembly requires that the sheets be clamped together prior to inserting the assembly within the chamber. Either simultaneously with the removal of air or subsequent to the initiation of the removal of air, the assembly within the chamber is heated to an elevated temperature, gas is introduced into the chamber in direct contact with the assembly at the elevated temperature to provide a superatmospheric gaseous pressure. The assembly is maintained at an elevated temperature in the chamber at a superatmospheric pressure only for a sufficient period of time to seal the layer to the glass sheets substantially throughout the entire area of the laminated assembly. The chamber is then cooled while maintaining the superatmospheric pressure until the bonded laminated glass assembly is cooled to a maximum temperature of about 150.degree. to 175.degree. F., the superatmospheric gaseous pressure is released in the chamber and the bonded laminated glass assembly is then removed.
The method described and claimed in the aforesaid patent to Morris has its drawbacks, mainly because of the fact that when vacuum is applied within a chamber, it is difficult to maintain uniform heat within the chamber. Also, since it is difficult to remove the clamps from the assembly while the latter is enclosed within a chamber, it was impossible to avoid severe marks where the clamps were applied to the assembly.
While the Morris patent does not use either prepressing rolls or laminating bags of flexible plastic, or peripheral rings that form a vacuum chamber in surrounding relation to the assembly to be preliminarily pressed, the poor control of heating within a vacuum chamber and the inability to limit the severity of clamp marks on the laminated assembly left room for improvement over the invention patented by Morris.